The demand for a holographic storage medium storing a plurality of data therein has been increased, wherein the holographic storage medium is required to have a high storage capacity.
In a typical system using a holographic storage medium therein such as a holographic ROM system, a reference beam and a modulated signal beam interfere with each other to generate a holographic interference pattern therebetween, wherein the modulated signal beam is generated by modulating the signal beam with a holographic signal of binary data to be recorded in the holographic storage medium. The holographic interference pattern may be stored in a disk-type holographic storage medium made of, e.g., an optical refractive crystal whose reaction on interference pattern depends on the amplitude and phase thereof. The modulated signal beam may be reconstructed by illuminating the holographic interference pattern with a reconstructing beam, of the same wavelength but with wavefronts that are “complex conjugate” (the reverse wavefront and direction) to the wavefronts in the reference beam, wherein the reconstructing beam is refracted through the holographic interference pattern to produce binary data.
In the meantime, a reflecting property is used to record a reflective signal of binary data in a reflective storage medium, e.g., DVD or CD, and to read the reflective signal from the reflective storage medium. In order to effectively use the reflecting property, it is preferable that the laser beam is vertically illuminated on the reflective storage medium. A cylindrical lens may be used to implement a focusing servo scheme on the reflective storage medium.
Since, however, the holographic storage medium uses the holographic property to record and reconstruct the holographic signal of binary data while the reflective storage medium uses the reflective property to record and read the reflective signal of binary data, a pick-up apparatus for a digital storage medium including the holographic and the reflective storage medium is required to have a down compatibility on the reflective storage medium.